A solar cell utilizing an organic metal perovskite crystal material (perovskite solar cell) can provide a high conversion efficiency. A large number of reports have recently been published on improvement on conversion efficiency of a solar cell utilizing a perovskite crystal material in a light absorbing layer (e.g., Non-Patent Document 1 and Patent Document 1). The organic metal used is a compound represented by a general formula R1NH3M1X3 (where R1 is an alkyl group, M1 is a divalent metal ion, and X is a halogen). Spectral sensitivity characteristics of the compound are known to vary depending on the halogen and/or the ratio of the halogen (e.g., Non-Patent Document 2).
A perovskite crystal material, such as CH3NH3PbX3 (X: halogen), can be used to form a thin-film at low cost using a solution application technique, such as spin coating. Thus, attention has been directed to a perovskite solar cell utilizing such a perovskite crystal material, as a low-cost and high-efficiency next generation solar cell. Furthermore, a perovskite solar cell has also been developed that incorporates, as a light absorbing material, CH3NH3SnX3 containing tin in place of lead (e.g., Non-Patent Document 3).
As shown in FIG. 8 (see Non-Patent Document 2), a perovskite crystal material exhibits a spectral sensitivity characteristic that is dramatically reduced at a wavelength of 800 nm, and thus absorbs little infrared light having wavelengths greater than 800 nm. Thus, to improve efficiency of a perovskite solar cell, it is important to effectively use long-wavelength light. For example, a combination of a perovskite solar cell and a solar cell having a bandgap narrower than that of the perovskite solar cell allows long-wavelength light to be used by the solar cell having a narrower bandgap. This is thought to achieve a solar cell with higher efficiency.
One known solar cell including a combination of multiple photoelectric conversion elements is a tandem solar cell, which is a stack of photoelectric conversion elements having different bandgaps. A tandem solar cell includes a photoelectric conversion element (front cell) having a wider bandgap provided on a light incident side, and a photoelectric conversion element (rear cell) having a narrower bandgap provided at the back side of the front cell. Since multiple photoelectric conversion elements are connected in series in the tandem photoelectric conversion element, effective drawing of photocurrents generated in the photoelectric conversion elements demands that the magnitude of photocurrents generated in each of the photoelectric conversion elements be identical.
In addition, a method has been proposed in which multiple photoelectric conversion elements having different bandgaps are disposed spatially spaced apart from each other, where a photoelectric conversion element having a narrower bandgap receives long-wavelength light, while a photoelectric conversion element having a wider bandgap receives short-wavelength light (e.g., Patent Document 2). This method eliminates the need for photocurrent matching from the multiple photoelectric conversion elements, thereby offers higher flexibility in design.